A conventional motor drive device carries out control by switching between drive under low-load conditions and drive under high-load conditions in order to extend a drive range of a brushless DC motor. That is to say, in low-load conditions, the conventional motor drive device carries out speed control based on a rotation position of the brushless DC motor by pulse width modulation (PWM) feedback control. Furthermore, a conventional motor drive device disclosed in Patent Literature 1 carries out synchronous drive that switches a phase to be energized of the brushless DC motor in a constant cycle in high-load conditions. FIG. 10 is a block diagram showing the conventional motor drive device described in Patent Literature 1.
In FIG. 10, power supply 101 is a general commercial power supply. In Japan, it is a 50 Hz or 60 Hz AC power supply having an effective value of 100 V. Rectifying and smoothing circuit 102 receives an input from AC power supply 101 and rectifies and smoothes DC electric power. Rectifying and smoothing circuit 102 includes four bridge-connected rectifier diodes 102a to 102d, and smoothing capacitors 102e and 102f. Inverter 103 includes six switching elements 103a to 103f which are coupled together via a three-phase full-bridge. Inverter 103 converts DC electric power from rectifying and smoothing circuit 102 into AC electric power, and supplies AC electric power of any voltage and frequency to brushless DC motor 104. Brushless DC motor 104 includes a rotor having a permanent magnet and a stator having three-phase star connected windings.
Counter electromotive voltage detection circuit 105 detects a relative position of the rotor from a counter electromotive voltage generated in a stator winding of brushless DC motor 104. Drive circuit 106 turns on and off switching elements 103a to 103f of inverter 103. Commutation circuit 107 decides which switching element of switching elements 103a to 103f is turned on based on the output from counter electromotive voltage detection circuit 105 when brushless DC motor 104 is driven in a stationary state. Synchronous drive circuit 108 outputs predetermined frequency and predetermined voltage (predetermined duty) when brushless DC motor 104 is driven as a synchronization motor.
Switch circuit 109 switches a signal to be sent to drive circuit 106 between a signal of commutation circuit 107 and a signal of synchronous drive circuit 108. PWM control circuit 110 carries out PWM control by chopping only switching elements of the upper arm or the lower arm of switching elements 103a to 103f of inverter 103. Output voltage is increased or decreased by increasing/decreasing a duty of a pulse width (the rate of the ON period in the pulse cycle). Load condition determination circuit 111 determines a load state of brushless DC motor 104 based on a signal from counter electromotive voltage detection circuit 105, and decides switching of a drive mode by switch circuit 109. Load condition determination circuit 111 includes first timer circuit 112, duty determination circuit 113, and phase determination circuit 114. First timer circuit 112 starts a timer when the drive by synchronous drive circuit 108 is started, and terminates the timer when a predetermined time has passed. Duty determination circuit 113 detects that the load reaches maximum when the duty reaches maximum (100%).
Phase determination circuit 114 detects a phase difference between a signal of counter electromotive voltage detection circuit 105 and a signal of synchronous drive circuit 108 to obtain a present load state. Frequency regulating circuit 115 detects a phase difference between the signal of counter electromotive voltage detection circuit 105 and the signal of synchronous drive circuit 108, and decreases the output frequency from synchronous drive circuit 108 when the detected phase difference is smaller than a predetermined value.
In the conventional motor drive device having the above-mentioned configuration, when a load to the motor is increased, a predetermined speed cannot be occasionally maintained by feedback control while detecting a rotor of brushless DC motor 104. Then, the conventional motor drive device switches the drive to synchronous drive by open loop control so as to be switched to commutation in which the target rotation speed is constant. After the drive is switched to the synchronous drive, the drive is returned to the feedback control again after a predetermined time measured by first timer circuit 112 has passed.
Thus, the rotor of the motor follows commutation timing with delay. In other words, a phase of a terminal voltage leads relative to a phase of an induced voltage, and similarly, a phase of a current is a leading phase relative to the phase of the induced voltage. Thus, the synchronous drive time becomes a state similar to magnetic flux weakening control. Therefore, it is possible to easily extend a drive range of brushless DC motor 104.
Even in a low torque motor that sacrifices the maximum rotation rate in order to improve the efficiency of the motor, a drive range is extended so as to obtain a desired rotation rate at the maximum load point. Moreover, under a usual load, a highly efficient motor can be driven with higher efficiency by feedback control.
However, in the above-mentioned conventional configuration, the brushless DC motor under high-speed and high-load conditions is driven synchronously by open loop in a constant commutation cycle. Therefore, in a predetermined low-load range, the induced voltage follows the applied voltage in a predetermined delayed phase according to the load. That is to say, the rotor of the brushless DC motor follows with respect to the commutation in a predetermined delayed phase. Furthermore, the phase of a current is determined from the relation between the induced voltage and the applied voltage. As a result, the phase relation of the induced voltage, the applied voltage, and a winding current of the brushless DC motor is stable in a predetermined state, and a load range in which drive can be carried out is extended.
However, when the load is a predetermined level or more, the rotor is delayed with respect to the commutation, resulting in a magnetic flux weakening state. That is to say, with reference to the position of the rotor, the phase of the applied voltage and the phase of the current become leading phases with respect to the phase of the induced voltage, resulting in a magnetic flux weakening state. In this state, the rotor is accelerated in synchronization with the commutation cycle. Thereafter, a lead angle of the phase of the current is reduced by the acceleration of the rotor, and this time, the speed of the rotor is reduced. This state is repeated, and the rotor repeats acceleration and deceleration. As a result, a drive state (a drive speed) may not converge into a stable state. That is to say, due to the change in the speed of the rotor, the phase of the induced voltage is unstable, and therefore the phase relation between the phase of the applied voltage and the phase of the current may be changed. In such a drive state, since the rotation rate of the brushless DC motor changes, a beat sound associated with the change in the speed may occur. Furthermore, there may be increase or decrease in the pulsation of a current of the motor by cyclic acceleration and deceleration, a stop of protection of over-current caused by the occurrence of the current pulsation, and finally, possibility of the out-of-synchronism of the brushless DC motor.
In order not to cause such problems, in conventional motor control devices, drive is not carried out in a state in which the rotation of a brushless DC motor is unstable. This means that the drive of the brushless DC motor under high-speed and high-load conditions is limited. In other words, there has been a problem that a drive range cannot be extended (that is, a drive range is narrow).